This invention relates to a pyroelectric camera tube.
A conventional pyroelectric camera comprises a sealed tube containing a target sheet of pyroelectric material. The sheet of pyroelectric material has a front surface which faces the scene to be viewed and which carries a front electrode known as the "signal plate". A rise in temperature of any particular part of the sheet results in a potential difference between opposite faces thereof. This is detected by scanning the rear face of the sheet with an electron beam and observing the resulting charge or discharge current to or from the front electrode.
Problems can arise because of thermal conduction from one particular spot on the pyroelectric sheet to the surrounding area. This reduces the resolution of the camera. A known method of reducing the severity of this problem is to etch a number of closely spaced crossed grooves into the pyroelectric material. The grooves present a barrier to heat conduction, thereby minimising loss of resolution. The technique of using these grooves is known as reticulation.
The grooves are etched by applying a first etch stop in the form of a pattern to the rear surface of the pyroelectric sheets, and then using an etching technique to etch grooves defined by the pattern. If it is desired to etch completely through the sheet it is useful to provide another etch stop in the form of a layer between the signal plate and the pyroelectric sheet to prevent the etching process penetrating the signal plate.
In order to ensure that the scanning electron beam provides an efficient read-out of the charge distribution on the pyroelectric material it is necessary to provide a positive bias on the rear surface. This bias is called the `pedestal` and can be produced in a number of different ways. One way is to introduce a gas into the tube which is ionised by the scanning electron beam. An ion current then serves to produce a positive charge or pedestal on the rear surface of the pyroelectric material. Another technique is to cause the electrons in the electron beam to accelerate to a high velocity during the line flybacks. In this way secondary electrons are removed from the pyroelectric target, leaving a positive charge. Yet another method is to apply a voltage ramp to the signal plate so as to provide, by capacitative coupling, a positive voltage on the scanned surface. Finally, a theoretical method has been proposed of providing the pedestal or positive bias by manufacturing the pyroelectric material in some way so that it is slightly conductive and applying a positive potential to the signal plate. This last technique has, however, never been commercially successful since, despite a considerable amount of research work, no satisfactory way of rendering the pyroelectric material slightly conductive has hitherto been found.